The present invention relates to a retinal image display device which emits a scanning light beam into an eye of an observer to form an image on a retina of the observer.
Conventionally, a retinal image display device which directly projects an image on a retina of an observer by scanning a weak light beam on the retina has been developed. The assignee of the present invention has suggested such a display device in Japanese Patent Publication No. 2874208. Such a retinal image display device is known as a head-mounting display, which is configured such that an observer wears the display device, like spectacles, on the head. The retinal scanning display device is implemented with a wavefront curvature modulator which dynamically varies the wavefront curvature of the beam in order to provide a depth of the image formed on the retina of the observer.
Light emitted by a light source propagates as a light wave in all directions at the same phase, i.e., as isophase spherical wave. Depending on a distance between the light source and an observer, the radius of curvature of the spherical wave at the observer is different. That is, if the light source is close to the observer, an image of the light source is projected on the retina of the observer as an image having a small radius of curvature, while if the light source is remote, the image of the light source is projected on the retina of the observer as an image having a relatively large radius of curvature of the wavefront. The observer recognizes the difference of the radius of curvature with the accommodation function (i.e., focusing function) of an eye and recognizes a natural perspective, or three-dimensional feel.
In the conventional wavefront curvature modulator, a frequency of the modulation which can be applied to the wavefront curvature is relatively small in comparison with a frequency of an intensity modulation applied to each pixel of the image, i.e., a video rate. Therefore, it has been difficult to achieve a variation of focusing condition providing a natural feel in a three-dimensional view.